Zamir V. Shomakhov

Metal–organic frameworks (MOFs) are new crystalline hybrid materials that have aroused great interest because of their growing applications in drug delivery, gas storage, and catalysis. The key limitation of pure MOFs for use in gas sensors is their low conductivity. Therefore, composites are often used. ZIF-8 improves the gas-sensitive characteristics of composites due to its hydrophobic properties, thermal stability, and high specific surface area. The purpose of this study was to design a hierarchical structure of ZnO nanowires coated with a porous ZIF-8 shell and to study its characteristics in detecting volatile organic compounds. ZnO/ZIF-8 composites were synthesized by the solvothermal method and annealed at 320 °C. Scanning electron microscopy revealed a net-like structure on the surface of ZnO nanowires. The gas analytical response was studied at 250 °C under the influence of acetone and ethanol vapors. The response and recovery times for the ZnO/ZIF-8 structure were found to be much longer in the presence of these gases compared to pure zinc oxide nanowires. The additional UV irradiation of ZnO/ZIF-8 provided an increased response and significantly reduced response and recovery times. UV irradiation accelerates the diffusion of gas molecules to the surface of ZnO through the ZIF-8 porous structure. This process leads to faster gas adsorption and desorption. Additional factors are photoinduced charge transfer and water desorption at the ZnO/ZIF-8 interface, contributing to the observed changes in the response.

Two-dimensional graphitic carbon nitride 2D g-C₃N₄ has the potential for gas sensing as a metal-free semiconductor with a layered structure, high surface area, and tunability of electronic properties. In this context, 2D g-C₃N₄ nanosheets were prepared by the thermal polycondensation of urea followed by ultrasonic exfoliation. X-ray diffraction revealed diffraction peaks corresponding to the (110) and (002) crystallographic planes of g-C₃N₄. Scanning electron microscopy showed a nanosheet structure with a 10-nm crystallite size, while energy-dispersive X-ray spectroscopy demonstrated a uniform distribution of carbon and nitrogen. Ultraviolet–visible absorption spectroscopy revealed a band gap of 2.8 eV. Gas sensing measurements exhibited an increase in response to isopropanol and ethanol as the operating temperature and gas concentration increased. Impedance spectroscopy provided additional insight into the sensing mechanism. Observed depressed semicircles in Nyquist plots were fitted with a charge transfer resistance R_ct in parallel with a constant phase element model. The charge transfer resistance R_ct fell systematically with isopropanol exposure, confirming the crucial role of adsorption-induced electron transfer in the gas sensing response.

Sensors based on mixed metal-oxide semiconductors are characterized by their fast response and low power consumption. Zinc oxide is widely used as a binary compound in these sensors. Gas-sensitive structures based on ZnO can be improved by creating nearby adsorption sites for the sensitizer gas and the detected gas. In this study, gas-sensitive layers based on zinc oxide nanorods are obtained by a hydrothermal method and then modified in solutions containing potassium stannate and iron sulfate as precursors to form ZnO–Sn and ZnO–Fe composite samples, respectively. Using X-ray photoelectron spectroscopy, the effect of the modification of sensor layers consisting of zinc oxide nanorods with subsequent treatment in solutions of compounds of other metals (tin and iron) on the chemical composition of the surface is analyzed. The sensor properties of the obtained samples are analyzed when detecting isopropyl alcohol vapors. It is found that depending on the technological conditions, the oxygen content in the form of adsorbed particles changes. Oxygen vacancies participate in the adsorption of these particles. It is shown that the gas-sensitive properties depend on the content of oxygen vacancies. It is established that the sensor layers are modified in the process of formation of ZnO–Sn and ZnO–Fe composite surface structures and leads to changes in the distribution of adsorption sites and their energy. The active oxygen and reducing gas adsorption sites are transformed with a change in the concentration of oxygen vacancies and leads to an increase in the sensor response to isopropyl alcohol vapor.

Introduction. Currently in gas sensing there is an urgent need to improve performance, including sensor response and speed, as well as to reduce operating temperatures. To achieve these goals, multicomponent oxide systems are being studied. Various modifications of Zn–Sn–O system are among the most promising materials.Aim. To develop techniques for the hydrothermal preparation of zinc hydroxostannate nanoparticles using different precursors. Active layers for gas sensors were created and tested using these synthesized nanoparticles.Materials and methods. Zinc hydroxostannate nanoparticles were synthesized using hydrothermal methods for 6 hours at a temperature of 90 °C. Two techniques using different precursors were used to obtain the samples. The samples were then analyzed using scanning electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy. Based on the optical spectroscopy results, the band gap of zinc hydroxostannate was determined. Gas sensing properties of the nanoparticles were investigated under different conditions, including target gases, temperatures, and exposure to UV radiation.Results. A comprehensive study of the synthesized structures has shown that the nature of their interaction with target gases is different depending on the synthesis technique. Specifically, ZnSn(OH)6_1 samples demonstrate a response at room temperature and are highly rapid. This is attributed to the predominance of zinc ions on their surface. In contrast, ZnSn(OH)6_2 samples, with a surface rich in tin ions surrounded by OH groups, require additional activation by UV radiation to achieve a response at room temperature. At the same time, the response and recovery times are in the order of hundreds of seconds.Conclusion. The paper explores the potential of using zinc hydroxostannate nanoparticles created through the hydrothermal process for room temperature gas sensors. The key factor in this study is the selection of the synthesis technique.

Currently, semiconductor gas sensors are being actively studied and used in various fields, including ecology, industry, and medical diagnostics. One of the major challenges is to reduce their operating temperature to room temperature. To address this issue, sensor layers based on WO_3−x/WS₂ nanostructures synthesized by the hydrothermal method have been proposed. In this paper, the morphology of the material’s surface and its elemental composition were investigated, as well as the optical band gap. Additionally, changes in the resistance of the WO_3−x/WS₂ sensor layers under the influence of alcohol vapors at room temperature were analyzed. The results showed that the layers exhibited a significant response, with short response and recovery times. The achieved response value to 1000 ppm of isopropanol was 1.25, with a response time of 13 s and a recovery time of 12 s. The response to 1000 ppm of ethanol was 1.35, and the response and recovery times were 20 s. This indicates that these sensor layers have promising potential for various applications.

Today, air pollution is a global environmental problem. A huge amount of explosive and combustible gas emissions that negatively affect nature and human health. Gas sensors are one of the ways to prevent this impact. Several types of sensors have been developed, but the main problem with them is the high operating temperature. This leads to a decrease in reproducibility and stability over time. The aim of the work is to synthesize zinc stannate nanoparticles, study their phase composition, and modify the structure with silver nanoparticles to improve gas-sensing characteristics. This paper presents the synthesis of zinc stannate nanoparticles by the coprecipitation method and annealing at varying temperatures. A method of decorating zinc stannate with silver nanoparticles was proposed. Using XRD, it was found that a change in the annealing temperature leads to a change in the initial phase composition of the sample. Decoration with silver nanoparticles allows for increasing the sensor response of zinc stannate layers to isopropyl alcohol by 20 times. The corresponding increase in response to ethanol is 16 times.

When developing gas sensors, selecting a suitable gas-sensing material is of great importance. Currently, there is great interest in the development of new gas-sensitive layers based on molybdenum oxide MoO₃ and improvement of their sensor characteristics. In the work, a MoO₃ gas-sensitive layer is obtained by the hydrothermal method followed by annealing at a temperature of 400°C for 2 hours. The elemental composition of the surface of the obtained sample is analyzed using X-ray photoelectron spectroscopy. Studies of the sensor response, response time, and recovery time of the gas-sensitive layer to isopropyl-alcohol vapors at different operating temperatures are carried out. The chemical composition is shown to be consistent with MoO₃, but adsorbed hydroxyl groups are also observed on the surface. Analysis of the sensor properties shows that the sensitivity at temperatures of 250°C and 150°C is 13.3 and 5.5, respectively. It is found that layers based on MoO₃ nanoparticles synthesized by the hydrothermal method exhibit sensitivity to isopropyl-alcohol vapors even at a temperature of 150°C.

Gas-sensitive devices have great potential for use in a variety of applications, including environmental monitoring, medicine, and various industries. The stability of gas sensors based on zinc oxide can be significantly affected by the presence of water vapors in the atmosphere. Gas-sensitive layers based on ZnO nanowires were synthesized by the hydrothermal method. The effect of different seed layers was studied in order to optimize the sensor properties of zinc oxide nanostructures. It was shown that layers consisting of zinc oxide nanowires synthesized using sacrificial doping on the seed layers of ZnO nanoparticles exhibited a moderate response to the vapor of volatile organic compounds, with almost no response to water vapor.

Introduction. Nanomaterials based on binary and multicomponent oxides are of interest for the development of catalysts, photocatalysts, gas sensors, solar cells, as well as in other fields. The most effective methods to produce oxide systems of various compositions are those of chemical co-deposition, as well as two-stage approaches.Aim. To develop sensor nanomaterials based on ZnO, Zn–Fe–O, and Zn–Sn–O ternary oxide nanosystems, as well as to develop methods for assessing their properties.Materials and methods. ZnO and ZnFe2O4 nanopowders were synthesized by chemical coprecipitation, and ZnFe₂O₄ and Zn₂SnO₄ nanostructures were produced by modifying ZnO nanowires. The surface chemical composition and microstructure were studied using scanning electron microscopy, backscattered electron diffraction, and Xray photoelectron spectroscopy. The sensor responses of the samples to vapors of organic solvents were analyzed.Results. The response value of zinc oxide and zinc ferrite samples synthesized by chemical coprecipitation was found to be 2–4 orders of magnitude higher than that of modified zinc oxide nanowires. The formation of ternary oxide nanostructures led to an increase in the sensor response of zinc oxide nanowires. This effect can be explained by the formation of adsorption sites of various types during formation of such systems. The samples produced by chemical coprecipitation showed an extremely high sensor response. This may be due to the formation of fractal structures at the percolation threshold.Conclusion. ZnO and ZnFe₂O₄ oxide nanostructures produced by chemical coprecipitation exhibit a high sensor response to acetone and ethanol vapors. Methods for the formation of multicomponent oxide systems with improved sensor properties compared to the original zinc oxide nanowires were developed. The resultant sensor nanomaterials are promising for use as sensitive layers of gas sensors for detecting organic solvent vapors.

Nowadays a significant issue lies in developing approaches to diagnosing the surface composition and structure of mixed oxide materials, including in local areas. The possibilities of X-ray photoelectron spectroscopy, energy-dispersive X-ray spectroscopy, and electron backscatter diffraction in the local region to study the surface composition of a hierarchical ZnFe₂O₄ nanostructure are shown. This nanostructure is synthesized by the post-treatment of zinc oxide nanowires in an aqueous solution of an iron salt. The studies show that zinc, iron, and oxygen are uniformly distributed over the surface of the sample. It is found that the electron-backscatter-diffraction pattern corresponds to the ZnFe₂O₄ compound. Using X-ray photoelectron spectroscopy, iron on the surface of the sample is shown to exist in the Fe³⁺ and Fe²⁺ states, and oxygen is present both in the crystal lattice and in the adsorbed state. We make an assumption regarding the processes that occur in the synthesis of a hierarchical structure that cause these properties. It is shown that the sensor signal of the hierarchical structure during the detection of isopropyl-alcohol vapor with a concentration of 1000 ppm^–1 exceeds the signal of zinc oxide nanowires.